Wireless backhaul has recently been growing in popularity with both fixed and mobile carriers, which ask for support of different classes of services for both real-time applications, such as voice, and non real-time applications, such as web browsing and video streaming. The former services require high levels of availability, while lower levels are envisaged for the latter.
In order to guarantee a certain degree of QoS and a certain level of service availability, wireless systems normally use some sort of radio protection scheme: the most commonly used radio protection is based on the principle of duplicating the radio link, and carrying the same traffic on both hops.
The two most common configurations for radio protection are 1+1 hot standby and 1+1 working standby:
Hot standby configuration is shown in FIG. 1 and consists of two transmitters 3a-3b and two receivers 5a-5b, tuned on the same frequency f1. Traffic 1 is split in two identical flows 1a and 1b and sent to two radio link indoor units 2a and 2b, and from there to the two transmitters 3a and 3b; only transmitter 3a is active, while transmitter 3b is in standby. Transmitter 3a sends the same data to receivers 5a and 5b, over the two channels 4a and 4b. Data is collected by two further radio link indoor units 6a and 6b, so that the resulting two traffic flows 7a and 7b are the same. The transmitter 3b transmits only if the transmitter 3a or the radio link indoor unit 2a experience problems.
In working standby configuration there are still two transmitters and two receivers, but they are tuned on different frequencies; the traffic is duplicated and both transmitters are active in parallel, sending the same data. FIG. 2 shows the most commonly implemented 1+1 working standby scenario: the traffic 1, split in two identical flows 1a and 1b, is sent to the transmitters 3a and 3b via the radio link indoor units 2a and 2b. Both transmitters are active and tuned on a different frequency f1 and f2. The traffic is then sent both by transmitter 3a to receiver 5a via channel 4a, and by transmitter 3b to receiver 5b via channel 4b. The received data is collected by the two radio link indoor units 6a and 6b, so that the resulting two traffic flows 7a and 7b are the same.
Most commonly, with respect to the need for protection, four different traffic typologies can be identified: circuit-switched traffic (typically voice), protected packet traffic, guaranteed packet traffic, which besides guarantees a maximum limit on delay, and best effort packet traffic, which carries non critical services such as web traffic or file transfers, and for which the network does not provide any guarantee that the data is delivered.
In recent years the technology of Adaptive Modulation and Coding (AMC) has been developed with the aim of improving the channel bandwidth efficiency.
The basic idea behind AMC is that the modulation and coding scheme on the communication channels is not fixed statically, but can vary dynamically over time in response to the varying quality of the radio link.
The use of AMC in combination with protection allows for a more effective exploitation of the available bandwidth. At present two main schemes that build upon the 1+1 working standby configuration are known.
The first solution consists in transmitting the same radio frames with the most spectral efficient modulation scheme on both links of the 1+1 configuration. This strategy maximizes the throughput in spite of protection, as the chosen best modulation might be not in accordance with channel condition of both links.
The dual solution prefers the protection in spite of the throughput by transmitting the same radio frames with the lower modulation scheme between the two links.
Known protection schemes do not consider any difference in the type of data traffic transported, that is, they protect all traffic, even the traffic that does not require protection such as guaranteed and best effort data, and this results in ineffective bandwidth utilization.